1. Technical Field
The present invention relates to a semiconductor memory device provided with a memory cell for storing and holding data. More particularly, the present invention relates to a technique to lower power consumption of a semiconductor memory device by cutting off a power supply to a specific circuit in the semiconductor memory device.
2. Background Art
Recently, a semiconductor process has been increasingly refined, which causes problems that a leakage current is increased in a transistor in a semiconductor memory device and power consumption is increased.
PTL 1 discloses a method for lowering power consumption while maintaining data stored in a memory cell when a semiconductor memory device is in a standby state. According to PTL 1, a leakage current is reduced and the power consumption is lowered by cutting off all power supplies to peripheral circuits while continuing a power supply to the memory cell. However, according to PTL 1, since all of the power supplies to the peripheral circuits are cut off, a word line is put into a floating state. Therefore, due to an influence of a switching noise of a power supply switch for cutting off the power supply to the peripheral circuit, there is a variation in potential of the word line in the floating state, and as a result, the problem is that the data stored in the memory cell could be destroyed.
PTL 2 proposes a configuration to solve the problem of PTL 1, in which a word line switch is added to fix a word line to a low impedance while power supplies to peripheral circuits are cut off.
In addition, PTL 3 discloses another method for lowering power consumption while a semiconductor memory device is in a standby mode. According to PTL 3, while the semiconductor memory device is in the standby mode (a frequency of accesses to a memory cell is 10% or less of a frequency in a normal mode), a bit line precharge circuit for precharging a bit line is controlled so that the bit line is put into the floating state, whereby a leakage current of the bit line precharge circuit can be reduced, and the power consumption can be lowered.